Almost all automobile electrical wire has two components. In focusing on the internal metal wire, I almost forgot the other component -- the insulating jacket, usually just called insulation.
The insulating jacket on wire is usually some form of flexible plastic. Solid copper wires tend to have a relatively rigid form of plastic, in part to keep the wire bending and flexing too much. On the other hand, stranded wire is intended to withstand repeated movement and bending so automotive wire (often just called auto wire) should have a more flexible, rubber-like plastic.
It's important for the insulating jacket to be continuous, without any cracks or gaps that can allow the electricity to "escape."
Regardless of the degree of flexibility, the plastics used for wire insulation all contain volatile compounds to help maintain flexibility. Over a span of decades, these compounds can leach out and evaporate, causing the insulation to become more rigid and brittle. So, very old wires are more likely to develop cracks and breaks in the insulation, making it much harder to ensure the electricity only goes where you want it to go.
The ambient temperature around the insulation has a direct bearing on how quickly the volatile compounds evaporate. Likewise, very low relative humidity tends to speed up the rate of evaporation, although to a lesser extent than high temperatures.
Very high ambient temperatures, such as in an engine compartment, can cause the insulation to degrade quickly, in as little as a few years. So, automotive wire (as well as marine wire) often has a temperature rating, using the Celsius scale. (On automotive wire, the temperature rating is often hard to notice or even missing. However, it's worth looking for the rating and avoiding any wire that doesn't have a rating, as well as any wire rated at 60 degrees C or below.)
The Blue Sea Systems online Circuit Wizard app includes a drop-down menu that allows you to select the wire insulation temperature rating.
It's worth noting plastic insulation, regardless of how flexible, gets stiffer as it gets colder. As long as it's not flexed significantly even severely subzero temperatures will not damage the insulation. (The main reason I prefer arctic grade wire is the insulation remains relatively flexible even at extremely subzero temperatures.)
There's nothing technically wrong with using wire with insulation rated at 105 degrees C (221 degrees F) exclusively. However, it costs a bit more and is a bit harder to work with because the insulation tends to be stiffer at normal outdoor temperatures.
1970 Explorer Class A on a 1969 Dodge M300 chassis with 318 cu. in. (split year)
1972 Executive Class A on a Dodge M375 chassis with 413 cu. in.
1973 Explorer Class A on a Dodge RM350 (R4) chassis with 318 engine & tranny from 1970 Explorer Class A
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Circuit protection is the intentional installation of a “weak link” in an electrical circuit.
(Source - Blue Sea Systems)Circuit protection is the intentional installation of a “weak link” in an electrical circuit.
CPDs are designed to interrupt a circuit when the electricity flowing in the circuit approaches damaging or dangerous levels. When the amount of amps in the circuit approaches the CPD's rating, the CPD opens (breaks) the circuit, stopping the flow of electricity.
CPDs are used in three basic ways:
1. To protect devices (loads), such as sensitive electronics, from being subjected to electrical current greater than they are able to handle. (Few things "make your day" quite like an expensive device emitting noise and smoke to indicate it's destined for the junk pile.)
2. To protect wire and connection components from an electrical overload that could cause them to heat up, melting and possibly igniting the insulation. In extreme cases, the generated heat could be enough to cause the metal wire to melt.
3. To protect from electrical short circuits.
The first two rarely happen in properly designed and well made circuits. When they do occur, it's usually due to either excessive resistance in the circuit or a short circuit somewhere.
An electrical short circuit is a shortcut across a circuit that provides an easier path for the electricity to follow back to the circuit source, bypassing the circuit load. The red line in the following diagram modifies our Basic Electrical Circuit diagram to demonstrate an electrical short.
![[image]](http://www.riffgan.com/motorhome/electrical/Electrical_Short_Circuit_1.jpg)
And, the following diagram shows the same thing in a common ground circuit like those found in motor vehicles. (I also added plus and minus symbols to more closely demonstrate the actual circuits.)
![[image]](http://www.riffgan.com/motorhome/electrical/Electrical_Short_Circuit_2.jpg)
There is a relationship between resistance and the amount of electrical current, described by Ohm's Law. (The Ohm's law formula includes circuit voltage but we can ignore that because we're dealing, at this point, exclusively with nominal 12VDC circuits.) In turn, based on the physics, there is an indirect relationship between wire size and resistance.
In a nutshell, smaller wire and higher current (amps) means more resistance. Conversely, larger wire and lower current means less resistance. At a certain point, moving from small to large and high to low, the resistance drops to point where it's negligible. (And the resistance curve effectively "flat-lines.")
So, electrical circuits are designed to use wire at least large enough to carry the expected current. In practice, slightly larger wire is preferred, with economic factors being the sole determination of how much larger is too large.
In addition to wire, connection components (wire terminals, screws, nuts, bolts, and so on) also create resistance within a circuit. Loose/corroded/dirty/etc circuit connections greatly increase the amount of resistance in a circuit. In response, more amps in the circuit are required to meet the demands of the circuit's load. This is exactly where well designed circuit usually go into overload conditions. (Even if this does not lead to catastrophic overload, problems with faulty circuit connections waste electricity, converting much of it to heat rather than actually powering the circuit's load.)
CPDs prevent catastrophe by serving as the "weak link" that fails before the rest of the circuit.
So, while wire should be sized for more that the expected current in the circuit, CPDs should be sized to match or, at most, slightly exceed the expected current.
The CPD's amperage rating should never exceed that of the wire and connection components. (Otherwise, the wire and devices become the circuit fuse.)
(Reliable companies that manufacture CPDs practice stringent quality control because of the legal liabilities involved in a CPD failing to perform as designed and rated.)
* This post was edited 06/26/17 11:39pm by Griff in Fairbanks *
In a properly functioning electrical circuit, the circuit load (electric lights, motors, devices, and so forth) effectively limit the current in a circuit to just what they need to operate.
In a short circuit, there is no limit. The short will try to cause the circuit to carry as much current as the circuit source can produce. In a circuit powered by a SLI (engine) battery, that current can be as much as 900 or more amps. For the most part, a 16 gauge automotive wire can only safely carry 12 amps. (Very short lengths of 16 gauge auto wire can carry more current, with voltage drop being more of a concern than current level -- due to Ohm's Law.)
Obviously, a wire meant to carry only 12 amps is going to go white-hot very quickly if subjected to 900+ amps. If you're very lucky, the wire will quickly melt and interrupt the circuit without causing additional damage. However, depending on what's near the wire, it could also easily start a fire.
A properly sized and installed CPD will prevent this from happening, blowing or tripping before all those amps can hit the wire.
In the American Boat and Yacht Council (ABYC) standards, each circuit must have a CPD within 7 inches of a circuit source or distribution point. There are a few exceptions, mostly for engine starter cables and main battery cables, but this is a good rule to follow.
Simply put, the closer a CPD is to a circuit's source, the more likely it is to stop an electrical short from creating a catastrophe.
Note: I prefer referencing and following ABYC standards for three reasons:
1. The environment where boats and yachts operate make it much, much harder to safely escape a fire, so the electrical standards have an added margin of safety written into them.
2. ABYC standards are routinely referenced is a wide variety of publications and websites, making it much easier to find guidance based on ABYC standards than that pertaining to motor vehicles. Likewise, the guidance is more thorough and definitive. (I often refer to and cite Blue Sea Systems literature but there are many more marine electrical system information sources that are just as good … and virtually all rely on ABYC standards.)
3. ABYC is a organization, like ISO and OSHA, that researches, develops, and publishes applicable standards. Rather than separately developing mandatory government regulations, applicable laws in many nations (including the United States and Canada) simply refer to ABYC standards and use those standards for enforcement.
The following is the most common symbol used to represent a fuse:
![[image]](http://www.riffgan.com/motorhome/electrical/Fuse_Symbol.jpg)
However, there's a wide variety of symbols used to represent fuses, circuit breakers, and just about every other type of electrical circuit component. Use whatever works for you. The important thing is you, and anyone needing to understand your circuit diagrams, knows what the symbols represent. For this reason, I prefer to use text to identify circuit elements, regardless of whatever symbols I'm using.
There are two basic types of CPDs, slow-acting and fast-acting. Slow-acting CPDs have a slight delay before tripping or blowing to handle anticipated brief current surges in appropriate circuits. Fast-acting CPDs are designed to trip or blow very quickly in response to current overloads and are much more common.
With very few exceptions, all CPDs in motor vehicles are fast-acting and are the only type commonly carried in parts stores and automobile departments in other stores. (The few slow-acting CPDs that may exist in motor vehicles all have unique configurations that keep them from being confused with, and used in place of, fast-acting CPDs or vice-versa.)
CPDs sometimes wear out due to age or repeated stress. (One way to "wear out" certain types of circuit breakers is to use them as on-off switches.) CPDs are designed to blow or stay tripped when they fail. So, it's reasonable to replace a blown fuse or reset a circuit breaker once. If the replacement fuse blows or the reset circuit breaker trips again, you have a problem with the circuit. (Repeatedly replacing fuses or resetting circuit breakers is a waste of time, effort, and money … the problem is something other than the CPD so dig in and find the actual problem.)
Finally, in older motor vehicles (like the motorhomes in this thread), there is a third type of CPD -- fusible links. I flat out hate these.
Like fuses, fusible links are single use protection and have to be replaced when they blow. Unfortunately, they look just like all the wires around them and, once you find them, you have to use some type of continuity tester to determine if they've blown.
Likewise, fusible links are very difficult to replace. Rather than copper, they contain a metal with a lower, specific melting point and their amperage rating is based on both their diameter and length. In addition to having to be sure to use the right size and length, trying to solder a replacement into a circuit often causes the link to melt. Furthermore, all but the very best crimp connection creates enough resistance to cause the replacement fusible link to blow. (For these reasons, just about all old-time mechanics hate them.)
Fortunately, reasonably priced high-amperage fuses and circuit breakers have become readily available, rendering fusible links obsolete. Whenever I run across a fusible link, blown or intact, I replace it with a modern fuse or circuit breaker.
You didn't mention the self-resetting circuit breakers that are in some of these old motor homes (maybe in newer ones too, I don't know). I hadn't run across these before I got my '79 RV. Kind of hard to find if you don't know they exist. Mine are small metal boxes with two screw terminals. They are attached directly to the house battery and I forget where they go to from there.
Comments?
For those interested in a more in-depth discussion, please see Wikipedia's Circuit Breaker page. (While good, be prepared for a discussion that gets very technical.)
People are most familiar with manual reset circuit breakers, like those found in home breaker panels. And, I'll admit sometimes using these as on-off switches when I really should install an additional, separate, heavy-duty switch.
There are also circuit breakers that automatically reset after a period of time. These are used in a variety of situations, such as inaccessible locations, no convenient place to mount a manual reset circuit breaker, anticipated brief current surges not requiring manual intervention/troubleshooting, and so on.
Internally, common circuit breakers use a thermal trip mechanism. These are most common because they are well suited to most situations, tripping quickly in response to significant overloads. (These are especially well suited to dealing with short circuits.)
(Automatic resetting thermal breakers were also used -- back in the days of vacuum tube radios, televisions, and computers -- to react to the device getting too hot, in addition to current overload protection.)
Other circuit breakers use an electromagnetic trip mechanism which reacts to extended periods of low level current overload that could lead to damage to the circuit components but don't always provide protection for significant overloads. (These are typically used in electronic devices.)
Furthermore, there are thermo-magnetic circuit breakers that react to extended periods of low level overload, as well as tripping quickly in response to significant overloads. These are well suited to both short circuit and loose/corroded/dirty/etc connection situations. (These tend to be much more expensive so I prefer to just use thermal circuit breakers and make sure all circuit connections are clean and tight.)
Most of this is only really a concern to people designing sophisticated, complex electronic circuits. For people seeking to maintain and improve their older motorhomes, the commonly available manual reset thermal circuit breakers are just fine.
* This post was edited 06/28/17 03:51pm by Griff in Fairbanks *
I suspect the one on TreeSeeker's coach battery was an aftermarket addition, because automatic reset circuit breakers weren't very common OEM equipment decades ago. They've become very common, and even ubiquitous, in late model vehicles. (Based on this, I believe a previous owner or mechanic was unusually proficient.)
Important note: If an automatic reset circuit breaker trips once or infrequently, there's probably no cause for concern. If, on the other hand, it trips regularly, you likely have a problem in your electrical system and should investigate.
I assume everybody is sufficiently familiar with switches and, to a lesser extent, relays so I'll just hit some of the aspects that tend to be confusing or less familiar.
(If you have questions regarding switches and relays, please PM me rather than asking via a thread post. I may already be planning to answer the question, may answer your question via a reply PM, or may quote your question and answer via a thread post if I think it is of interest to a wider audience.)
Switches and relays are categorized as normally open (N.O. or NO) and normally closed (N.C. or NC). This designation is especially important for momentary switches and most relays. If the designation is missing from the description, you can assume the switch or relay is N.O.
The meaning of the two terms is simple. If the designation is N.O., it means the switch or relay, when not activated (pressed or energized), "opens" the circuit and prevents electricity from flowing through the circuit. N.C. means the opposite, the switch or relay, when not activated, "closes" the circuit and allows electricity to flow through the circuit.
Switches and relays usually have another designation in the form of xPxT, with both x replaced with a S, D, or occasionally a number. (For example, SPDT, DPST, 3PST, etc.)
The xP refers to x poles, which is a fancy way of saying x circuits. SP is single pole, meaning the switch or relay controls only one circuit. In turn, DP is double pole, meaning the switch or relay can simultaneously control two separate circuits. (3P means simultaneous control of three separate circuits and so on.)
The xT refers to switch or relay positions. ST is single throw, meaning a simple ON-OFF switch like most of the light switches in your house. Just as with poles, DT is double throw, meaning an ON-ON switch or relay. (Be careful, ON-ON switches are sometimes labeled as ST and ON-OFF-ON switches are technically 3T switches but are often erroneously labeled as DT switches.)
Multiple position switches (3T, 4T, etc.) are common. However, almost all relays are either ST or DT, due to technically difficulties in making multi-position relays and a general lack of need for relays with multiple positions. (Furthermore, electronic circuits can easily be created that satisfies the need should it arise.)
So, momentary SPST N.O. is the complete technical description for a simple pushbutton switch.
Note: N.O. and N.C. is important in momentary switches, with the designation being meaningless in all other switches that are not "spring-loaded." On the other hand, the designation is important for relays, especially xPDT relays to identify which terminals are normally open and which are normally closed.
In addition to simply turning a circuit on or off, switches and relays can be used to create complex circuits involving a degree of logic, just like simple computer circuits. For example, I have a circuit using a relay to light up a bright red LED if the headlights are on when the engine's not running. The pseudo-code statement is: IF NOT (engine running) AND (headlights on) THEN (turn on LED). (Buzzers/chimes are annoying so I'm using a LED instead.)
The following are typical symbols for common varieties of switches.
![[image]](http://www.riffgan.com/motorhome/electrical/Switch_Symbols.jpg)
Just as with the symbol for fuses, use whatever symbols work for you, making sure your diagrams can be understood by others, if necessary, and yourself long after you're done working on the circuit. (I can't count the times I've gone back to a diagram -- or piece of software code -- years later and tried to recall what everything meant … or what the **** I was thinking when I designed it.)
Because OFF-ON tends to be more understandable, many vendors use that rather than SPST or DPST. In those cases, you will often see OFF-(ON), with the parenthesis indicating momentary contact. For example, in a boat with both automatic and manually controlled bilge pumps, people often use an ON-OFF-(ON) switch. (For those who also own boats -- don't nitpick, this is a simplified example to demonstrate a possible practical application for a less common type of switch.)
So, OFF-(ON) is a simpler and clearer N.O. momentary.
There is one type of switch that on the surface appears to have no practical application. It's a double pole OFF-OFF-(ON) / OFF-ON-(ON) switch and is actually used as a OFF-RUN-START switch for engines such as in a genset. The first pole -- OFF-OFF-(ON) -- is for energizing the starter and the second pole -- OFF-ON-(ON) -- for energizing the motor's ignition system. In the "middle" position, the ignition system is on and the starter is not. In the "right," momentary position (opposite the OFF position) both the starter and ignition circuits are energized.
Note: We'll get into it later, when we're done with the Coach System and shift to the Motor Vehicle System, but right now it's worth mentioning the switch in the preceding paragraph (above) is essentially the same as the key switch in your motorhome. The difference is your key switch has a fourth position -- ACC -- for energizing some motor vehicle circuits (such as the radio) without energizing the engine's ignition.
It's important to first consider the logic (i.e., what you want the circuit to do and when) before trying to find switch (and relay) configuration that does what you need.
Also, most people tend to assume the +12VDC supply connection should be made to the center terminals on a xPDT switch but it's often valid to do the opposite. The following diagrams demonstrate this.
![[image]](http://www.riffgan.com/motorhome/electrical/Switch_Examples.jpg)
The top example illustrates using a single source to exclusively power two separate circuits and the bottom example shows exclusively using two separate sources to power a single, combined group of circuits.